Vinyl alcohol polymers (hereinafter, may be abbreviated as “PVAs”) typified by polyvinyl alcohol are known as water-soluble synthetic macromolecules, and are widely used in intended usages including raw materials for vinylon, which is a synthetic fiber, paper coating agents, fiber coating agents, adhesives, stabilizers for emulsion polymerization and suspension polymerization, binders for inorganic substances, and films. In particular, PVA is superior in strength characteristics and film-forming property as compared with other water-soluble synthetic macromolecules. Accordingly, based on these properties, compositions containing the PVA are successfully utilized as coating agents for improving surface properties of substrates such as papers (clear coating agents, binders in pigment coating, and the like), and the like.
In order to further enhance such characteristics of the PVA, various types of modified PVAs have been developed. As one example of the modified PVAs, silyl group-containing PVA is exemplified. The silyl group-containing PVA has superior water resistance and binder force to inorganic substances. However, the silyl group-containing PVA is accompanied by the following disadvantages that: (a) in the preparation of aqueous solutions thereof, the silyl group-containing PVA is less likely to be sufficiently dissolved unless an alkali such as sodium hydroxide or an acid is added; (b) viscosity stability of the prepared aqueous solutions tends to readily deteriorate; (c) in the formation of a coating film containing an inorganic substance, it is difficult to obtain a coating film having the water resistance and the binder force to the inorganic substance both being satisfactory; and the like.
In light of the foregoing, there have been proposed a silyl group-containing PVA having an improved solubility in water and the like by adjusting a product (P×S) of a viscosity average degree of polymerization (P) and a percentage content (S: mol %) of monomer units having a silyl group to fall within a predetermined range, and the like (see Japanese Unexamined Patent Application, Publication No. 2004-43644), and a coating agent containing the silyl group-containing PVA (see Japanese Unexamined Patent Application, Publication No. 2005-194437). However, in the silyl group-containing PVA, the upper limit of the product (P×S) has been proposed to be 370, and a trade-off relationship between enhancing the characteristics as the silyl group-containing PVA through an increase of the percentage content of the monomer units having a silyl group and enhancing a solubility in water and the like thereof has not been overcome. More specifically, the silyl group-containing PVA has a disadvantage in handling that in a case where the product (P×S) is 370 or greater, in preparing an aqueous solution of the silyl group-containing PVA, it is occasionally impossible to dissolve the silyl group-containing PVA unless an alkali or an acid is added, as disclosed in paragraph 0009 of Japanese Unexamined Patent Application, Publication No. 2004-43644 cited above. Accordingly, the silyl group-containing PVA also has not sufficiently solved the aforementioned disadvantages.
Furthermore, more superior water resistance (including boiling water resistance) or blocking resistance may be required for coating films produced from a PVA-containing composition, depending on various types of modes of use and the like. Accordingly, development of compositions capable of providing a coating film that meets these characteristics has been desired.